In a mouse model of endometriosis, PCB-126 exposure significantly promoted the growth of ectopic lesions by activating the Steroid Receptor Coactivator-1 (SRC-1) isoform/Matrix Metalloproteinase-9 (MMP9)/Estrogen Receptor-β (ERβ) axis, a key driver of disease progression. The gene discussed is MMP9; the disease is endometriosis.